1. Field of the Invention
The present invention relates to electroless plating of rhodium onto a substrate. More particularly, this invention pertains to an aqueous rhodium plating bath, a process for plating a uniform coating of rhodium onto various substrates using an electroless plating composition, and a rhodium plated article formed therefrom.
2. Description of the Related Art
Plating of metals is a well known process employed to alter the existing surface properties or dimensions of a substrate. For example, a substrate may be plated for decorative purposes, to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to a substrate. Plating is a common practice in many industries, including the manufacture of a variety of electronic packaging substrates, such as printed circuit boards.
There are various methods of plating known in the art, including electroplating and electroless plating. Electroplating involves the formation of an electrolytic cell wherein a plating metal represents an anode and a substrate represents a cathode, and an external electrical charge is supplied to the cell to facilitate the coating the substrate.
Rhodium is employed as a thin coating on substrates, in electronics, optics, fuel cells, electrical contacts, automotive catalysts, gas sensors, corrosion protection, insoluble electrodes, gas turbine engines, X-ray mirrors, supported catalysts, jewelry, medical implants and many other applications. Metallic rhodium (Rh) provides the unique combination of excellent physical and chemical properties such as hardness, wear resistance, corrosion resistance, electrical and thermal conductivity, optical reflectance, catalytic activity and others.
Rhodium coatings are typically deposited by electroplating, vacuum deposition (sputtering), or chemical vapor deposition (CVD) techniques. Vacuum deposition of rhodium coating is expensive, requires complicated equipment and cannot be done uniformly on geometrically complicated substrates. Deposition of rhodium coating by the CVD process is expensive, requires complicated equipment and cannot be done on temperature sensitive substrates like electronic components or polymer membranes for fuel cells because of the CVD method requires thermal decomposition of vapors of rhodium chemical compounds on substrate surface at 200+xc2x0 C.). CVD rhodium coatings are contaminated with 2-35% carbon. (See A. Etspuler, et al., xe2x80x9cDeposition Of Thin Rhodium Films By Plasma-Enhanced Chemical Vapor Depositionxe2x80x9d, Appl. Phys., A, 1989, v. 48, pp. 373-375). Also, deposition of CVD coatings on powders is not easy.
Rhodium electroplating is carried out from highly acidic sulfuric or phosphoric electrolytes (See R. Morrissey, xe2x80x9cRhodium Platingxe2x80x9d, Plating and Surface Finishing 1999, v. 86, No. 11, p. 62; and Modem Electroplating, Edited by F. Lowenheim, John Wiley and Sons, NY, 1974, pp. 350-352). Electrically non-conductive substrates like polymer membranes for fuel cells and ceramic monolithic supports for automotive catalysts cannot be plated. Non-precious metals can be properly plated only after pre-plating with nickel or precious metals because of high acidity of rhodium electrolyte. Geometrically complicated substrates cannot be uniformly plated because of poor distribution of current density in the rhodium electrolyte. Plating on powders is practically not available. Also, sulfur and phosphorus from the electrolyte contaminate an electroplated rhodium coating. Rhodium purity is critical for a number of applications like corrosion protection and catalysis. It has been shown that the presence of 0.01-0.001% sulfur and phosphorus and chlorine in a noble metal corrosion protective coating reduces its protective action and the lifetime of gas turbine components by 25% (See U.S. Pat. No. 5,788,823).
Electroless plating involves the deposition of a metallic coating from an aqueous bath onto a substrate by a controlled chemical reduction reaction which is catalyzed by the metal or alloy being deposited or reduced. This process differs from electroplating in that it requires no external electrical charge. One attractive benefit of electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Frequently, electroplating an irregularly shaped substrate produces a coating having a non-uniform deposit thickness because of varying distances between the cathode and anode of the electrolytic cell. Electroless plating obviates this problem by excluding the electrolytic cell. Another benefit of electroless plating over electroplating is that electroless plating is autocatalytic and continuous once the process is initiated, requiring only occasional replenishment of the aqueous bath. Electroplating requires an electrically conductive cathode and continues only while an electric current is supplied to the cell. Also, electroless coatings are virtually nonporous, which allows for greater corrosion resistance than electroplated substrates.
In general, an electroless plating bath includes water, a water soluble compound containing the metal to be deposited onto a substrate, a complexing agent that prevents chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate, and a chemical reducing agent for the metal ions. Additionally, the plating bath may include a buffer for controlling pH and various optional additives, such as bath stabilizers and surfactants. The composition of a plating bath typically varies based on the particular goals of the plating process. For example, U.S. Pat. No. 6,042,889, teaches an electroless plating bath and having a hypophosphite reducing agent and employing one of several different xe2x80x9cmediator ionsxe2x80x9d, including rhodium ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Rhodium exhibits good hardness, catalytic, friction and wear properties, but present application techniques are very expensive. Known efforts to plate rhodium onto substrates have been difficult because of problems in producing a pure metallic rhodium coating on a substrate and undesirable degradation of the aqueous bath.
U.S. Pat. No. 4,361,602, teaches a process for electroless plating of rhodium onto a substrate using a aqueous composition, or plating bath, comprising a rhodium salt, a nickel salt as an activating agent and an alkylamine borane wherein the molar ratio of nickel salt to rhodium salt ranges from 0.01 to 2.0.
The disclosed process uses rhodium as a chloride, bromide, perchlorate or a sulfate. This process is disadvantageous in that the presence of nickel, boron, carbon or chloride in the plating bath are non-volatile compounds that lead to impure rhodium plating. Also, the accumulation of non-volatile byproducts and non-consumable elements from the plating bath inhibit replenishment of the plating bath, leading to undesirable termination of the plating process.
Japanese patent JP58204168, teaches a process for electroless plating of rhodium onto a substrate using an aqueous plating bath comprising a rhodium as a chloride, hydroxyl amine salt as a stabilizer and hydrazine as a reducing agent. This process is also disadvantageous because the accumulation of non-volatile chloride byproducts and non-consumable elements from the plating bath lead to an impure rhodium plating and inhibit replenishment of the plating bath, leading to termination of the plating process. Additionally, Swiss patent CH 606475 and German patent DE 2607988 teach a process for electroless plating of rhodium with similar problems resulting from non-volatile chloride and sodium byproducts.
The present invention solves the problems of the prior art by employing a process for electroless plating of rhodium using a composition comprising an aqueous solution comprising a rhodium ammine nitrite salt, ammonium hydroxide as a complexing agent and hydrazine hydrate as a reducing agent. The composition of this aqueous solution is free of non-volatile components that cause impure plating, allowing for improved appearance and properties of the plated rhodium. Further, the process generates essentially no hazardous substances and the absence of non-volatile components avoids the accumulation of byproducts that degrade the plating bath, allowing for virtually unlimited replenishment of the bath. Moreover, the unique composition of the plating bath allows metallic rhodium to be precipitated from the plating bath by boiling without undesirable contaminants.
This invention provides a simple low-cost method of a deposition of ultra pure rhodium coatings on virtually any material of any geometrical shape, including fibers and powders, by electroless autocatalytic plating. The method involves the controlled autocatalytic chemical reduction of a rhodium salt by a chemical reducer with the formation of a dense uniform metallic rhodium coating of unlimited thickness selectively on the substrate surface which is contacted with a rhodium plating bath.
The invention provides an electroless plating composition comprising an aqueous solution comprising:
i) triamminetris(nitrito-N,N,N)rhodium(III);
ii) ammonium hydroxide; and
iii) hydrazine hydrate.
The invention further provides a process for plating a substrate comprising:
a) providing a plating composition comprising an aqueous solution comprising:
i) triamminetris(nitrito-N,N,N)rhodium(III);
ii) ammonium hydroxide; and
iii) hydrazine hydrate; and
b) contacting a substrate with the plating composition for a sufficient time and under conditions sufficient to plate metallic rhodium onto the substrate.
The invention also provides a process for plating a substrate comprising:
a) providing a plating composition comprising an aqueous solution comprising:
i) triamminetris(nitrito-N,N,N)rhodium(III);
ii) ammonium hydroxide; and
iii) hydrazine hydrate;
b) immersing a substrate into the plating composition for a sufficient time and under conditions sufficient to plate metallic rhodium onto the substrate; and
c) removing the substrate from the plating composition.
The invention further provides an article comprising a substrate immersed in a composition comprising an aqueous solution comprising:
i) triamminetris(nitrito-N,N,N)rhodium(III);
ii) ammonium hydroxide; and
iii) hydrazine hydrate.
The present invention provides a process for uniformly plating various substrates with metallic rhodium using an electroless plating bath. Initially, an aqueous plating bath comprising water, triamminetris(nitrito-N,N,N)rhodium(III), ammonium hydroxide and hydrazine hydrate is formed in a suitable container. Triamminetris(nitrito-N,N,N)rhodium(III), CAS Registry Number 102129-82-8, can be formed, for example, by reacting K3[Rh(NO2)3Cl3] with ammonia (NH4). The preferred complexing agent is ammonium hydroxide (NH4OH). The preferred complexing agent is a 28% solution of ammonium hydroxide. The preferred reducer is a hydrazine compound, most preferably hydrazine hydrate (N2H4.H2O). Other suitable hydrazines include hydrazine chloride and hydrazine sulfate, but are not preferred because of the greater probability that rhodium will precipitate out of the bath. Accordingly, the preferred plating bath can be described by the following general formula:
[Rh(NH3)3(NO2)3]+0.75 N2H4.H2O=Rh+3.75 N2+6.75 H2O
The amount of the triamminetris(nitrito-N,N,N)rhodium(III) present in the bath preferably ranges from about 0.01 to about 320 g/L. More preferably, the amount of triamminetris(nitrito-N,N,N)rhodium(III) ranges from about 0.01 to about 5 g/L. The amount of 28% ammonium hydroxide present in the bath preferably ranges from about 1-1000 mL/L, more preferably from about 10 to about 500 mL/L. The preferred amount of hydrazine hydrate present ranges from about 0.01 to about 240 g/L, more preferably from about 0.1 to about 10 g/L.
Once all of the components are combined in a suitable container, the water soluble triamminetris(nitrito-N,N,N)rhodium(III) dissolves, releasing rhodium ions into the bath. The ammonium hydroxide complexing agent forms a strong complex with the rhodium ions and prevents chemical reduction of the rhodium ions in the bath while permitting selective chemical reduction on a substrate surface. The hydrazine hydrate reducer allows reduction of the complex rhodium ions at the substrate surface. In particular, after a substrate is immersed in the plating bath, the substrate surface catalyzes oxidation of the reducing agent. This oxidation causes a release of electrons that, in turn, reduce metal rhodium ions in the bath at the substrate surface. These reduced metal ions are then deposited onto the substrate and, over time, generate a metal shell around the substrate. The hydrazine hydrate reducer allows reduction of the complex rhodium ions to metallic rhodium which is deposited selectively on a substrate surface because of a catalytic action of a substrate surface. Chemical reduction of the triamminetris(nitrito-N,N,N)rhodium(III) by hydrazine hydrate results in the formation of only metallic rhodium and highly volatile gaseous byproducts (N2, H2O and NH3) which are removed from the plating bath by evaporation. Other bath constituents, including N2H4.H2O, are also highly volatile and can be similarly removed through evaporation. The bath contains no substances capable of accumulating in the container and suppressing the rhodium plating process, and creates no hazardous substances. The plating composition is highly stable and does not require the addition of non-volatile stabilizers, accelerators, pH regulators or other chemical agents used to enhance plating properties. Also, because no strong complexing agents are included or generated by the bath, simple boiling of the bath is sufficient to precipitate virtually pure rhodium from the aqueous solution.
The process is autocatalytic, in that no catalyst separate from the aforementioned components is required to advance the rhodium deposition on a catalytically active surface like base and noble metals, alloys, graphite and others. Catalytically inactive materials like glass, ceramics and polymers can be activated by conventional methods, for instance by contacting with a tin salt solution and/or a noble metal solution. Additionally, the process is continuous and may be maintained for virtually an infinite time by merely replenishing each of the components of the bath.
The bath is maintained at a temperature ranging from about 20xc2x0 C. to about 98xc2x0 C., more preferably from about 60xc2x0 C. to about 90xc2x0 C. The bath is also maintained at a preferred pH ranging from about 8 to about 13. Preferably, the bath is formed in the absence of any other additives since such would tend to accumulate in the bath. These conditions are important factors in maintaining a stable plating bath and preventing precipitation of rhodium from the bath.
Following formation of the plating bath, a suitable substrate is immersed in the bath for plating. The substrate remains in the plating solution for a time sufficient and under conditions sufficient to plate a substantially uniform coating of metallic rhodium of required thickness onto the substrate. Usually the plating rate is about 0.1 to 2 micron/h. It increases with increasing temperature and concentration of rhodium and hydrazine. Typically the substrate remains in the plating bath for from about 1 minute to about four hours depending on the required rhodium thickness preferably from about 5 minutes to about 60 minutes and most preferably from about 5 minutes to about 30 minutes . After the desired amount of metallic rhodium has been coated on the substrate, it is removed from the plating solution. The result is an article having a substantially uniform and virtually pure metallic rhodium plating, having good appearance and properties. Plating can also be done by contacting a substrate surface with a plating bath by any other technique such as spraying, pouring, brushing, etc. The substrate may comprise any material ranging from non-metals, metals, alloys, semiconductors and non-conductors. Suitable metal substrates include stainless steel, carbon steel, nickel, iron, chromium, iron-chromium alloys, and nickel-chromium-iron alloys. Suitable non-metals include printed circuit boards, polyimide substrates, ceramic and glass substrates.
The type of container used to form the plating bath is also an important factor affecting the stability of the bath. In particular, the container should be non-metallic to prevent reduction of the metal ions on the walls of the container. Additionally, means used to heat the bath should be a non-metallic heating system, and should heat the bath uniformly to prevent any reductions of metal ions in the bath.